Separation of pure bark fiber from finely comminuted bark



Aug. 10, 1948. R PAULEY 2,446,551

SEPARATION OF PURE BARK FIBER FROM FINELY COMMINUTED BARK Filed Oct. 19,1945 mscoez FEET/C41.

Comes:

HAMMER MILL ROBERT D. PAULEY INl ENTUIZ Patented Aug. 10, 1948 ,UNlTEDSTATES PATIENT orrlcs' SEPARATION or PURE BARK FIBER FROM FINELYcoMMINU'ranBAaK e Robert D. Pauley, Weyerhaeuser Longview, Wasin,assignor to Timber Company, Longview, Wash, a corporation of WApplication October 19, 1945, Serial No. 623,251

'1 Claims. (crane-235) l This invention relates to the treatment of barkand has particular reference. to an improved method of treating Douglasfir bark to produce a substantially pure and clean bark fiber fraction.

tially filled with air and of low specific gravity.

The constituents of the phloem are longitudinal cells comprising sievetubes, frequently bast fibers or stone cells, phloem parenchyma, andradial cells comprising ray parenchyma.

It has been discovered that by adjusting the friability of the barkmatrix, a suitable comminuting process such as ball milling maybeemployed to differentially puiverize the constituents of the bark tobreak the bond therebetween and so permit separation thereof byselective screening. Such separation is productive of various classes ofmaterials, each of which may be recovered as a fraction substantiallyfree from mixture or contamination.

The first of these materials is the phellem or cork developed each yearby the phellogen or cork cambium. The phellem or cork consists ofnonelongated thin and thick walled cells converted into cork tissue bydevelopment of suberin. The thin walled cells probably are not heavilysuberized and tend to collapse in the radial direction. The cork cellsare partially filled with air and are of low specific gravity.

The second class of materials to be found in certain barks, Douglas firin particular, comprises a tough fibrous portion of the phloem known asbast fiber. This material is resistant to destruction by millingprocesses and is recovered in the form of individual hard fibers. Forpurposes of the instant disclosure, this material is referred to as barkfiber, and largely comprises a fraction having a coarseness intermediatebetween the cork fraction and the bark powder fraction. At low moisturecontent these fibers still tend to maintain their identity, even whensubjected to the severe pulverizing action by which other portions ofthe phloem are reduced to powder.

A third class of materials is that herein classified as bark powder,being composed essentially of sieve tubes and other portions of thephloem exclusive of the bast fibers. This material readily disintegratesunder the action of a ball mill at a moisture content at which the corkand bark fiber fractions are highly resistant to disintegration.

A method of separating and recovering these three materials is disclosedin an application for United States Letters Patent, Serial No. 556,097,filed September 27, 1944, now Patent No. 2,437,-

672 dated March 16, 1948, in the name of Herman W. Anway and havingcommon assignee with the instant application. The present inventionrelates in general to the method therein disclosed and has as its objectto provide an improved method of treating Duogia's fir bark to produce apure bark fiber fraction.

In experimenting with the formulation of molding compounds employingbark products, it is found that each of the three classes of materialsabove mentioned imparts certain definite characteristics to articlesmolded therefrom, and that different characteristics maybe obtained atwill by using one or more of these materials in variant proportions inthermosetting molding compounds. In order to standardize and reproducesuch formulations it is necessary, therefore,

to separate these materials into relatively pure fractions. The presentinvention is concerned only with the fiber fraction. The bark powder andcork fractions are also produced and separated in the course of theprocess.

The method of the invention will now be described with'referen'ce to thedrawing, illustrating diagrammatically the functional steps in theprocess.

In the practice of the invention, bark may be stripped from logs andchopped or otherwise broken up to facilitate feeding the bark to ahammer mill or other suitable grinding equipment for converting the barkinto a substantially uniform mass of bark particles. Prior to thegrinding step it is desirable to wash the bark for removing therefromsand, dirt and other extraneous matter. During or after the washingoperation the chopped bark is preferably screened, the screen openingsbeing selected to pass and eliminate smaller particles of rotted barkand foreign materials. The principal purpose of the grinding step is toso reduce the particle size of the bark as to provide for control of itsmoisture content in a continuous process of producing the several barkfractions. Further purposes of the grinding step are to prosingle figureof the 1 particles of rotton bark,

In the grinding step t e raw whole bark Ill,

washed clean of undesira le matter, is fed into a hammer mill II whichbreaks up the pieces of bark and discharges its product through a screenof approximately 54" mesh. The product of the hammer mill is, therefore,a massof' bark particles l2 of fairly uniform size andconsistingapproximately of'the same constituents which compose the whole bark,although some of the phloem constituents will be found to be finelycommi-' nuted during the grinding operation, The bark is then in asuitable condition for the rapidad- Justment of the moisture content ofits several constituents for subsequent operations.

The first drying action occurs as the ground bark I2 is conveyed throughducts bythe action of a hot air blower l3 which deposits it in acollection chamber M. The collection chamber ll may preferably be of thecyclone type wherein the stream of air from the blower i3 is separatedfrom the flow of bark particles and exhausted to the atmosphere. Thisstream of heated air may, however, be conducted to the blower I: forreuse in conveying and. drying the bark.

From the collection chamber H the material is fed at a controlled feedrate onto an inclined vibrating screen l5 which may preferably be a 65mesh screen having square openings .0082 inch on a side. The finematerial which passes through the screen l5 comprises primarily barkpowder. with whichv the present invention is not concerned. The removalof the fines in the first screening step increases the total output ofthe system by decreasing the load on the ball mill- II which receivesthe material passing over the screen l5.

4 screen, and serves to remove the balanc of the fines after the ballmill, thereby reducing the amount of material to be handled in the finalscreening steps. The product It passed through the second screencomprisesprimarily bark powder and may be combined with the product ofthe screen I! for a useful purpose. The presence of fines and corkparticles on the screen l1 congests the screen openings and preventsmuch of the best fiber from passing through the screen, and the controlof the feed rate to effect this mode of operation is an importantconsideration in the practice of the invention. The screen congestionhowever does not block. the fines from passing therethrough.

The material passing over the screen I! comprises cork particles ofvarious sizes, and bast fibers. This mixture is then fed to larger meshscreen I9, which may preferably be a 28-mesh screen having squareopenings, .0232" on a side, for separating out the oversize +28-meshmaterial 20. The material which passes through the screen I! containsthe fiber fraction along with The ball mill i6 effects a controlledpulverizing action to finely comminute the friable portion of thematerial fed thereto, the time of treatment being in part determined by'the moisture contem t the material. The action of the ball mill breaksthe bonds between sieve tubes and other portions of the phloem, and thecork, leaving bundles of best fibers as a first stage in the comminutionstep. The time of treatment is adjusted, however, so that the bundles ofbest fibers are broken up and yield individual fibers in the dis-rcharge of the ball mill along with the fine bark powder and coarser corkparticles. In order to obtain the desired comminuting action in areasonable time, the moisture content of the material leaving thecollection chamber l4 must be such that any further drying action on theway to the ball mill IE will produce the desired'moisa ture content inthe material by the time it arrlves at the ball mill, Further dryingtakes place during milling, depending upon the time of treatment, andall these variables must be coordinated by sampling. On a typicalproduction run the moisture content of the ground bark was found to bereduced by the drying system from 42.6% to 16.3%, oven dry basis. 'Amoisture content of 15% to 25%, based on the dry weight of the bark, isdesired.

The second screen ll may also be a fifi-lnt hcork particles of a sizewhich passes over the screen I? and through the screen i9, this materialconstituting a relatively smah proportion of ing means on the oppositeside edges. A rigid bar is attached to the screen throughout its lengthalong a medial line parallel to and midway between the tensioned sideedges. This bar is actuated by a vibrator mechanism (not shown) to causethe screen to vibrate like a diaphragm at right anglesto the plane ofthe screen.

The fibers constituting the ultimate fraction to be separated are rigidslender spindles having a length of roughly ten diameters, the diameterbeing somewhat less and the length being considerably greater than thescreen openings of screens l5, l1 and 2| so that they will not passthrough such a screen when they are lying fiat.-

This accounts for the fact that all the material reaching the screen 2|has previously passed over the similar screens l5 and II. On the screens15 and II the feed rate is such that with the bulk of other materialpresent relatively few of the fibers have the freedom necessary to upendthemselves and slip lengthwise through the openings to appear in theproduct -I 8.

A unique action takes place on the screen 2|.

With apslow feed rate and a relatively small amount of other materialpresent to congest the screen openings, the ultimate individual fibershave suiilcient freedom to respond to the vibration by dancing orjumping on the screen until they are perpendicular tothe screeningsurface so that they can drop longitudinally through the openings, thefibers being too long to pass through when lying fiat on the screen. Thefine mate-- rial having been eliminated by the screens l5 and H, the endproduct 22 i a substantially pure fiber fraction.

In practice, the screens l9 and 2| may be combined to form a double deckscreen unit producing oversize +28-mesh cork and fiber product 20, acoarse 28 +65-mesh cork and fiber product 23, and the -65-mesh purefiber fraction 22. Average percentages of these three products atpresent recovered from the screens i9 and 2i in relation to the totalproducts produced-from thebark are asfollows:- 1 g In an operation usingapparatus such as is herein described. the -.85 fiber fraction 22 isfound to contain about 6% cork, or, in other, words, this fraction'isproducedat. 94% purity. For use in molding compounds and the like, thisfraction may be considered as pure fiber, and is herein designated aspure orsubstantiaiiy pure fiber.

In order to obtain this degree of fiber purity in the product 22, muchfiber is lost in the other products. The product i8 ordinarily, containsfrom 20% to 35% fiber. product 20 contains from 85% to 60% fiber, andproduct 23 contains from 10% to 80% fiber. The disposition or subsequenttreatment of these products is immaterial to the present invention, butthey are all useful and important products obtained from the operationof abark plant using the disclosed process. The amount of fiber lost inthe products It, 20 and 23 is subject to a certain amount of control byadjustment oi the different variables affecting screencongestionconditions such as feed rate,

screen vibration rate, and moisture content, as

Per cent +65 mesh 7.2 65 +100 mesh 50.4 100 +200 mesh 40.4 -200 mesh e0.9

In interpreting the above percentages, it must be remembered that thescreening techniques in a sample analysis are entirely difierent fromthe screening actions accomplished by the automatic vibrator screens ina production plant of the type above described. Thus, it might beexpected that the 7.2% of +65 mesh material would have been removed bythe screen 2| so that it could not possibly get into the product 22, andit might be expected that the 0.9% oi 200 mesh material would have beeneliminated in the product l8, but such is not the case. Likewise, the-100 +200 mesh sub-fraction of 40.4% might be expected to appear in theproduct it rather than in the prodnot 22. This apparent paradox,however, is explained by the factthat the fibers are crowded intohaphazard positions in a mass of other material in passing over thescreen l1, and in the short time allowed by the high feed rate they areso obstructed that they cannot drop freely through the screen, eventhough they may become up- The ultimate individual fibers of Douglas firbark are hard and tough and seem-to sufl'er very little from the actionof' the hammer mill and ball -mill; The'main effect of this'treatmen'tupon the fiber is the breaking up ofthe-fiber bundlesto free theindividual'fibers. Fibers taken from the product 22 and viewed under themicrosc'ope appearto have '-a spindle shape, and vary in size from .016to .051 inch in length and from .0016 to .0055 inch in diameter at themid-section. While they showvery little'evidence of the roughtreatmentthey have received, some of the fibers do appear to have becomebroken into short lengths and some to be partly broken or bent, as astick of wood may be broken without separating the two ends. The shortends of broken fibers are probably lost in the material i8, the'longerends segregating themselves with the rest of the fiber fraction 22. Thebent or partly broken fibers would have verygreat diiii'c'ulty, though,in passing through the screen 2|, and would account for a part of thefiber component of the product 23. The amount of fiber'breakage is, ofcourse, sublect to a certain amount of control in the ball mill ii. Ifthe-fiber bundles receive insufilcient treatment in the ball mill theywill not be broken up to free the ultimate individual fibers and thefiber bundles themselves will consequently'pass over the screens i0 and2| and appear in the products 20 and 23. On the other hand, if themoisture content were very low,'and it the feed rate were so slow thatthe treatment in the ball mill was unduly prolonged, undoubtedly theamount of fiber breakage would be greatly increased. As has beenpreviously stated, the success of the present process depends largelyupon the coordination of different variables to obtain a novel screeningaction wherin the ultimatefiber fraction will pass over a particularscreen in one step and then pass through an approximately identicalscreen in a subsequent step to achieve the substantially completeelimination of all other fractions. The term "ultimate fiber is used todistinguish from bark fiber bundles.

No limitation is intended by reference to a particular type of apparatusfor practicing the method of the invention, the same being limited only,by the scope of the appended claims. In particular, applicant does notwish to be limited to the specific apparatus referred to for performingthe grinding and comminuting steps, nor does applicant wish to belimited to specific sizes of screen openings or specific fiberdimensions, except as may be specified in certain claims. References inthe specification to particular apparatus and screen openings are madeby way of illustration only, to describe a preferred manner of carryingout the steps of themethod in a continuous process best suited for aproduction plant.

Other apparatus for grinding andcomminuting may be devised by thoseskilled in the art and some variance may be made in the screeningapparatus by properly coordinating the variables ended from time totime.- Because the fiber mentioned in the specification, and all suchmodi-, fications and variations are included in the invention.

- Having now described my invention and in what manner the same may beused, what I claim as new and desire to protect by Letters Patent is:

1. Themethod'of producing substantially pure bark fiber comprising dryscreening in a plurality of successive screening operations finelydivided bark containing loose ultimate fibers having lengthssubstantially greater thantheir diameters, the screens employed in thefirst and final screening operations having mesh openings of a sizegreater than the diameter and less than the length of the fibers,maintaining a congested layer of material on the screen during the firstscreening operation to restrain the fibers irom passing through thescreen, maintaining a relatively thin layer of material on the screenduring the final screening operation and vertically vibrating thematerial in said final screening operation to up-end the fibers andpermit them to pass through the screen while rejecting oversizematerial.

2. The method of producing substantially pure bark fiber from finelydivided bark containing loose ultimate Douglas fir fibers havln'glengths substantially greater than their diameters, comprising dryscreening said finely divided bark in successive screening operations,the screen employed in at least two of said screening operations havingmesh openings of a size greater than the diameter and less than thelength of the fibers, maintaining a congested layer of material on thescreen during the 'first screening operation to restrain the fibers frompassing through the screen while removing the fines, maintaining a,relatively thin layer of material on the screen during a subsequentscreening operation and vertically vibrating the material in saidsubsequent screening operation to up-end the fibers and permit them topass through the screen while rejecting oversize material.

3. The method of producing substantially pure bark fiber from a mixtureof whole Douglas fir bark comminuted so as to consist of cork particles,ultimate fibers having lengths substantially greater than theirdiameters, and a finely pulverized non-fibrous portion of the phloem,comprising successively passing said comminuted mixture in the dry overscreens having mesh openings of a size slightly greater than thediameter but substantially less than length of said fibers, feeding thematerial to a first screen at a rate to maintain a congested layer ofmaterial thereon to restrain the fibers from passing through said screenwhile removing the fines, feeding the overs to a subsequent screen at arate to maintain a relatively thin layer of material thereon to enablefreedom of movement of the loose fibers, and vertically vibrating thematerial on said subsequent screen to cause said fibers to up-end andpass through said screen while the oversize fraction of cork particlesis being rejected.

4. In the method of producing substantially pure Douglas fir bark fiberfrom a finely comminuted bark mixture containing loose ultimate Douglasfir fibers having lengths of from .016 to .051 inch and diameters offrom .0016 to .0055 inch, the steps of successively dry screening saidmixture over two screens having approximately .0082 inch mesh openings.the first screening step being conducted while maintaining a congestedlayer of material thereon to restrain the fibers from passingtherethrough while removing the fines, the second screening step beingconducted while maintaining a relatively thin layer of material thereonto permit the fibers freedom of movement, and actuating said secondscreen to cause said fibers to up-end and pass therethrough whilerejecting oversize material as overs.

5. The method of producing substantially pure bark fiber from finelydivided bark containin loose ultimate fibers having lengthssubstantially greater than their diameters comprising dry screening saidfinely divided bark in three successive screening operations, each ofthe screens employed in the first and third operation havlnl meshopenings of a size greater than the diameter and less than the length ofthe fibers and the screen employed in the second operation having meshopenings of coarser size, maintaining a congestedlayer 0! material onthe screen during the first screening operation to restrain the fibersfrom passing through said first screen, passing the overs from the firstscreen to the second screen to reject during the second operation acoarse traction of material while passing therethrough a substantialportion or the fibers, maintaining the throughs from said secondscreening operation in a relatively thin layer on the screen during thethird screening operation and vertically vibrating the material in saidthird screening operation to up-end the fibers and permit them to passthrough said third screen while rejecting oversize material. V 6. Themethod of producing substantially pure bark fiber from finely dividedwhole Douglas fir bark containing loose ultimate fibers having lengthssubstantially greater than their diameters, comprising dry screeningsaid finely divided bark in a plurality of successive screenin steps,the screen employed in th first and final steps having mesh openings ofa size greater than the diameter and less than the length of the fibersand the screen employed in an intermediate step having a mesh opening ofcoarser size, maintaining a congested layer 01' material during saidfirst screening step to restrain the fibers irompassing through thescreen, passing the overs from said first screening step to the screenemployed in said intermediate step to remove a coarse fraction ofmaterial while passing therethrough a substantial portion of the fibers,maintaining the throughs from said intermediate screening step in arelatively thin layer on the screen in said final screening step andvertically vibrating the material in said final screening operation toupend the fibers and permit them to pass through the screen whilerejecting oversize material.

7. The method of producing substantially pure bark fiber from a mixtureof finely divided Dou las fir bark containing cork particles, looseultimate fibers having lengths of from .016 to .051 inch and diametersof from .0016 to .0055 inch, and a finely pulverized portion of thephloem, comprising successively passing said finely divided mixture inthe dry over a plurality of screens, the first and final screens being65 mesh and an intermediate screen being 28 mesh, maintaining acongested layer of material on the first screen to restrain the fibersfrom passing through, said screen while removing the fines, passing the.

overs from said first screen to said intermediate screen to reject acoarse fraction of cork particles, maintaining the throughs from saidsecond screen in a relatively thin layer on the final screenv REFERENCESCITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Name Date 77,829 Miller May 12, 1868 378,516Oliver Feb. 28, 1888 (Other references on following page) Number NumberI 10 MATES mm FOREIGN PATENTS Rune Date Number Country Date 25,198Netherlands Sept. 16, 1931 8:3 3; 2'1932 280,721 Great Britain Nov. 24,1927 Dennlng 1 Oct. 27, 1936 OTHER REFERENCES g gg g:- :82; Industrial81 Engineering Chemistry, Aug. 1944,

pp. 759-764. P11111198 1942 Wise-Wood Chemistry, Reinhold Pub, 00., Beck1943 N. Y., 1944, pp. 940. Smith June 1, 194a Johnson Nov. 7, 1944

